Method of making porous metal bodies



Feb 1942- R. G. 0LT 2,273,589

METHOD OF MAKING POROUS METAL BODIES Filed March 7, 1940 INVENTOR A;ATTORNEYS Patented Feb. 11, 1942 METHOD OF MAKING POROUS METAL BODIESRichard G. Olt, Dayton, Ohio, assignor to General Motors Corporation,Detroit, Mich., a corporation of Delaware Application March 7, 1940,Serial No. 322,753

18 Claims.

This invention relates to a method of sintering metal powders and isparticularly concerned with the sintering of metal powders having aconstituent rich surface thereon.

It is an object of the invention to provide a method for making highlyporous metal articles capable of permitting fluid flow therethrough,wherein the porosity of the articles may be closely controlled andwherein the shrinkage during sintering may be substantially eliminated.

Another object of the invention is to provide a method for making porousmetal objects wherein a porous alloy article is produced from powdershaving a constituent-rich surface thereon.

In carrying out the above object it is a further object to providepowders which contain at least two constituent metals, one of saidmetals being rich on the surface of the other metal, that is, each metalpowder particle will contain a core of one metal and have a surface richin another metal. It is a still further object in some cases in carryingout this object to mechanically attach highly comminuted particles of alow melt ing constituent metal to the surface of larger particles of ahigher melting constituent metal.

A further object in some cases is to use powders wherein the constituentrich surface is prealloyed to a core or nucleus of another metal wherebythe relative positions of the constituents are fixed before sintering indefinite positions with one another.

Another object of the invention is to provide porous metal articleshaving a substantially uniform degree of porosity wherein the strengthof the article is relatively high and wherein the finished article maybe a homogeneous allo throughout if desired.

Further objects and advantages of the present invention will be apparentfrom the foilowing-description, reference being had to the accompanyingdrawing wherein a preferred embodiment of the present invention isclearly shown.

In the drawing:

Fig. 1 is an illustrative view showing highly magnified particles ofmetal powders 20 having a low melting constituent metal 22 mechanicallyheld to the surface of the high melting constituent 22 prior to thesintering thereof.

Fig. 2 is a view showing the particles as shown in Fig. 1 aftersintering thereof.

Fig. 3 is a view of particles of metal powder having a constituent richsurface that have been pre-alloyed prior to sintering.

Fig. 4 is a view of particles in Fig. 3 after sintering the metalthereof.

Fig. 5 is a fragmentary sectional view showing a highly porous metallayer 32 bonded to a steel supporting surface 34.

The use of non-compacted metal powders as mentioned hereinafter isdisclosed in Patent No. 2,157,596 and in Patent Nos. 2,198,253,2,198,702 and copending application Serial No. 253,596, while the use ofbriquetted materials is shown in the Williams Patent No. 1,556,658, allof which are assigned to the assignee of the present invention.

The present invention is directed to a method whereby the porosity of analloy article may be closely controlled, the strength increased and thealloying action of the constituent metals to form a more homogeneousarticle accomplished in a shorter time. In the present invention Ipropose to attach a low melting constituent metal directly to thesurface of a high melting constituent metal whereby the distribution ofthe constituents are substantially uniform throughout the unsinteredarticle, thereby preventing excessive shrinkage and simultaneouslyproviding for uniform and rapid alloying. It is further apparent thatthe porosity of the finished article will be substantially uniform andmay be closedly controlled.

The attachment of the low melting constituent metal to the highermelting constituent metal may be accomplished in several manners, forexample, copper powder, which is used for illustrative purposes only,may be considered the high melting constituent noted at 24 in Fig. 1that is used wherein the mesh size thereof is preferably about 150. Thiscopper powder 25 is mixed with a binder and a desired quantity of lowmelting constituent 22 such as tin powder preferably having a mesh sizeof about 300. In other words,

the low melting constituent 22 should be considerably smaller in size ofparticle than the high melting constituent 24 although the particularsizes noted are not limiting. The binder may be lacquer, oil or liquidplastics, etc., in small quantities, or some other type of binder whichwill cause the low melting constituent to uniformly adhere to thesurface of the high melting constituent. Binders are well known and ithas been discovered that in some cases even water may be used. Thepowders are mixed for a suitable period and are then removed anddistributed in a non-compacted condition within a mold on a supportingsurface and then sintered. The binder is preferably added to the highmelting constituent 24 and coated thereover prior to the addition of thelow melting constituent 22. The volatile binding medium is dissipated atthe temperature of sinterin'g, leaving the low melting constituent 22 inclose proximity to, and in the desired relative position with, the highmelting are not held in a definite relation to one another prlor tosintering.

' The invention as described herein is particuconstituent 24, whereupondiffusion takes place with uniformity. It is apparent thatthemechanically held layer. of low melting constituent which theparticles are pre-alloyed for a short period, thereby causingsuperficial diiiusion of the tin, for example into the copper. In thisembodiment the copper particles 30. are coated with tin 28 which isbonded metallurgically to the copper and forms a constituent richsurface. Upon sintering, the tin further difluses into the copper andalso bonds adjacent particles by an alloy bond.

While the foregoing description has been directed primarily to copperand tin particles, it is apparent that other metals may be used, forexample, copper-nickel, wherein the nickel is 'the high meltingconstituent and the copper forms the constituent rich surface layer,nickeltin, wherein the tin is the low melting 'consti: tuent forming thelayerover the nickel particles. Similarly, iron-copper may be used withthe copper forming the low melting constituent. It is alsov possible touse constituent rich surfaces on metal particles wherein the surfaceitself is an alloy or a mechanical mixture of metals, for example,copper particles may be coated with tinantimony-lead, or lead-antimony,likewise the high melting constituent may be an alloy, for example, theparticle may be abronze which is coated with tin-antimony, or tinwherein after sintering the tin percentage of thebronze is in-' creased.Likewise if iron is used as the high melting constituent, the iron mayinclude nickel,

manganese, carbon, etc., or any other suitable alloying ingredients.These particles may-be.

coated witha low melting constituent, for example, copper. Thus, theinvention is directed to the utilization of a constituent-rich surfacewhich surface acts as the low melting constituent of the final alloy tobe produced and the f particular metals or alloys form no part of theinvention since such metals depend upon the desired final product. 1 gIn all cases it is preferred to use spherical particles to produceuniformity and greater porosity in the finished article. It is apparentthat spherical particles in a non-compacted conditionalthough any of theother types of metal powders may be provided with constituent-richsurfaces by methods heretofore described and thus pro-'- duce articlesfrom non-compacted metalpowders having greater strength than thoseproduced wherein the particles of the constituent metals particles ofdiiferent constituents having diflarly adapted for reventing-se regationor the 7 metal powders, for example, when tin powder and copper powderare mixed the constituent metal powders due to the difierence in densitytendtosettleinlayers. 'l'hisisparticularlytrue wherethereisadiiferenceinthemeshsizein the powders or where powders'are usedwhich have a variable mesh size which is often the cases In theseinstances the fines tend to segregate from the coarser particles andsimilarly the ferent densities tend to segregate in layers. This s reation is due to vibrations of the supporting plate. By following theteachings of this invention itis possible due to the attachment of thelow melting constituent to the particles of high melting constlhients,to prevent segregation by the fixing of consfltuent metals in de- 'siredrelative positions conducive to rapid and 05 toward the nucleus of thecopper particles to a copper-rich constituent and possibly at the centeroi. each particle to pure copper. This,- of course, depends upon thetimeof sintering and the desired amount of alloying to be accomplished.

,While the foregoing disclosure is directed to procedures using theloose non-compacted powders it is apparent that metal powders of thetype described may be advantageously utilized "when resorting tomolding, such as hriquetting,

and it is to be understood thatthe invention is of sumciently broadscope to hev used in connection with any of the well known manufac-While the embodiment of the present invention as herein disclosed,constitutes a preferred form, it is to be understood that other formsmight be adopted, all coming within the scope of the claims whichfollow.

' What is claimw is as follows:

1. In a method for reducing the sh ge of porous alloy articles duringsinteringwhich articles are made from non-compacted metal powder, thesteps comprising: providing metal powder having at least two metallicconstituents of different melting points, particles of said powderincluding a constituent rich surface thereon, said constituent being thelowest melting constituent and then sintering the powder in the loosenon-compacted condition and in desired shape under suitable conditionsat a temperature above the melting point of the constituent rich 7surface metal and below the melting point of the melting constituentmetal for a time suificient to cause at least partial alloying of thelower melting constituent with the other constituent whereby the powderis bonded together in an alloy bond without excessive shrinkage.

2. The method as claimed in claim'l wherein the metal powder having aconstituent rich surface thereon consists of high melting point metalhaving relatively smaller grains of lower melting point metalmechanically attached to the surface thereof.

3. The method as claimed in claim 1 wherein the metal powder having aconstituent rich surface consists of grains of metal powder alloywherein the outer surface comprises a lower melting point constituentthan does the inner portion thereof.

4. The method as claimed in claim 1 wherein the constituent rich surfaceis predominantly tin and the higher melting point metal is rich incopper.

5. The method as claimed in claim 1 wherein the constituent rich surfaceconsists predominantly of copper and the higher melting point metal ispredominantly iron.

6. The method as claimed in claim 1 wherein the constituent rich surfaceis predominantly copper and wherein the higher melting point metal ispredominantly nickel.

'7. In a method for controlling shrinkage of porous alloy articlesduring sintering thereof which articles are made from non-compactedmetal powder, the steps of fixing the relation of the alloy constituentsso that the low melting point constituent is in a relatively thin layerover the higher melting point constituent and then sintering the powderin a non-compacted condition and into the desired shape whereby uponsintering of the powder the diffusion of the metals, as occasioned byalloying, causes a relatively slight change in position of theparticles.

8. A method of forming highly porous alloy articles comprising the stepsof; providing metal powders having a constituent rich surface thereonwherein the constituents are components of the final alloy desired,distributing a layer of said powder upon a supporting surface in theloose non-compacted condition, and then sintering the layer undernon-oxidizing conditions at a temperature above the melting point of theconstituent rich surface metal but below the melting point of the metalin the nucleus of the particles for a time suflicient to cause theconstituent rich surface metal to diffuse uniformly into each particle,thereby bonding the particles together by an alloy bond for forming astrong homogeneous article without excessive shrinkage, said articlehaving a substantially uniform and controlled porosity.

9. In the method of forming highly porous articles the steps ofproviding metal powder particles including at least two component metalsI which are fixed in definite relation to one another whereby thelowmelting constituent is adjacent the surface of the particles,distributing a layer of said powder upon a supporting surface in theloose non-compacted condition, and then sintering the layer upon thesupporting surface under non-oxidizing conditions at a temperature abovethe melting point of the low melting component metal for a timesufiicient to cause diffusion of the low melting component into theother component metal whereby the particles of metal powder are bondedtogether by an alloy bond for forming a strong layer without excessiveshrinkage, said article having substantially uniform and controlledporosity.

10. In the method of forming highly porous articles the steps of;providing metal powder particles including at least two componentmetals, which are fixed in definite relation to one another wherein thelow melting constituent is adjacent the surface of the particles,filling a mold with said powder in the loose non-compacted condition.heating the mold with the powder therein under non-oxidizing conditionsat a temperature above the melting point of the low melting constituent,but below the melting point of the other component metal for a timesufficient to cause the lowmelting constituent to diffuse into the highmelting constituent for bonding the metal particles together by an alloybond thereby forming a strong homogeneous article without excessiveshrinkage and with a controlled and high porosity, and then removing thearticle so formed from the mold.

11. In the method of forming highly porous articles the steps ofproviding metal powder particles including at least two componentmetals, which are fixed in definite relation to one another wherein thelow melting constituent is adjacent the surface of the particles,loosely molding said powder into the desired shape, and then heating themolded powder under non-oxidizing conditions at a temperature above themelting point of the low melting constituent, but below the meltingpoint of the other component metal for a time sufficient to cause thelow melting constituent to diffuse into the high melting constituent forbonding the metal particles together by an alloy bond thereby forming astrong article of substantially uniform controlled porosity and withoutexcessive shrinkage.

12. In the method of forming highly porous alloy articles comprising thesteps of; providing metal powder having a constituent rich surfacethereon wherein the constituents are components of the final alloydesired, distributing a uniform layer of said powder upon a non-adheringsupporting surface in a loose non-compacted condition, heating the layerof powder upon said surface under non-oxidizing conditions at atemperature above the melting point of the constituent rich surfacemetal but below the melting point of the metal in the nucleus of theparticles for the time sufficient to cause the constituent rich surfacemetal to diffuse uniformly into adjacent particles thereby bonding theparticles together by an alloy bond and forming an article of goodstrength and with a controlled substantially uniform porosity withoutexcessive shrinkage, and then removing the article from the supportingsurface.

13. In the method of forming highly porous alloy articles comprising thesteps of; providing metal powder having a constituent rich surfacethereon which consists of a lower melting constituent than the nucleusof the powder, said constituent being attached to each particle of thepowder mechanically, distributing the uniform layer of said powder upona non-adhering supporting surface in a loose non-compacted condition,heating the layer of powder upon said surface under non-oxidizingconditions at a temperature above the melting point of the constituentrich surface metal but below the melting point of the metal in thenucleus of the particles for the time sufficient to cause theconstituent rich surface metal to diffuse uniformly into each particle,thereby bonding the particles together by an al- 10y bond and forming anarticle of substantially uniform high porosity without excessiveshrinkage, and then removing the article from the supporting surface.

14. In the method of forming highly porous alloy articles withoutexcessive shrinkage during sintering comprising the steps of providingmetal powder particles having a constituent rich surface thereoncomprising a lower melting constituent than the nucleus of theparticles, said lower melting constituent being metallurgically bondedby prealloylng to the remainder of each particle, distributing theuniform layer of said powder upon a non-adhering supporting surface in aloose non-compacted condition, heating the layer of powder upon saidsurface under non-oxidizing conditions at a temperature above themelting point of the constituent rich surface metal but below themelting point or the metal in the nucleus of the particles for the timesufficient to cause the constituent rich surface metal to diffuseuniformly into each particle, thereby bonding the particles together byan alloy bond and forming an article of controlled and substantiallyuniform porosity and of good strength, and then removing the articlefrom the supporting surface.

15. In the method of forming highly porous articles without excessiveshrinkage during sintering comprising the steps of; providing copperpowder having a high tin layer covering the surface thereof,distributing said powder upon a dition, and then sintering the layerundernontering comprising the steps of; providing nickel powder having athin layer of copper covering the surface thereof, distributing saidpowder upon a supporting surface in a. loose non-compacted condition,and then sintering the layer under non-oxidizing conditions at a.temperature above the melting point of copper and below the meltingpointof nickel for a. time sumcient to cause the metals to difiusewhereby coppernickel bonds are formed between adjacent particles forforming an article of substantially uniform and controlled porosity andof good strength.

17. Inthe method of forming highly porous alloy articles withoutexcessive shrinkage during sintering comprising the steps of providingprealloyed powders wherein the-extent of pre-alloying is limited to thesurface thereof so that the surface of each particle is rich in a lowmelting constituent, distributing said powder upon a supporting surfacein a loose non-compacted condition, and then sintering the layer undernon-oxidizing conditions at a temperature above the melting point of lowmelting constituent and above the melting point of the remainder of themetal particle for a time sumcient to cause the oxidizing conditions ata temperature above the has a controlled and substantially uniformporosity and is of good strength;

16." In the method of forming highly porous articles without excessiveshrinkage during sin-;

constituents in the powder to further difi'use whereby alloy bonds areformed between adjacent particles for forming an article ofsubstantially uniform and controlled porosity and of good strength.

18. The method as claimed in claim 9 wherein the porous layer is bondedto the supporting surface. 1

RICHARD G. 0LT.

